Eliminate legacy gateway container (consolidate into router-gateway)

The gateway service was a redundant deployment of the same
StellaOps.Gateway.WebService binary already running as router-gateway.
It served no unique purpose — all traffic is handled by router-gateway
(slot 0). This removes the container, its route table entries, nginx
proxy blocks, health/quota stubs, and redirects STELLAOPS_GATEWAY_URL
to router.stella-ops.local so the Angular frontend resolves API base
URLs through the canonical frontdoor.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

devops/compose/openapi_routeprefix_smoke_microservice.csv

@@ -75,7 +75,6 @@
 "Microservice","/jwks","https://authority.stella-ops.local/jwks","/","200"
 "Microservice","/authority","https://authority.stella-ops.local/authority","/authority/audit/airgap","401"
 "Microservice","/console","https://authority.stella-ops.local/console","/console/filters","401"
-"Microservice","/gateway","http://gateway.stella-ops.local",,
 "Microservice","/scanner","http://scanner.stella-ops.local","/scanner/api/v1/agents","401"
 "Microservice","/policyGateway","http://policy-gateway.stella-ops.local","/policyGateway","302"
 "Microservice","/policyEngine","http://policy-engine.stella-ops.local","/policyEngine","302"